High molecular weight polyesters prepared from diacids and dihydric alcohols have found extensive use in a variety of commercial products. Their excellent ductility and flexibility have found utility in fibers for apparel, carpeting, and tire cord. In addition, they are widely used in thin film applications such as magnetic tape and product packaging.
In recent years, high molecular weight polyesters have found increasing use as metal coatings, particularly in applications requiring a high degree of extensibility such as when coated metal sheeting is formed by drawing into cans or can ends that are used to package foods. However, these polymers suffer from a lack of solvent resistance since only their end groups are reactive with crosslinking agents. This fact, coupled with the very short oven bake conditions used in these types of applications, lead to very low levels of crosslinking in the baked film and poor resistance to solvents, particularly to those solvents that swell the film.
This lack of solvent resistance is addressed in European Patent Applications 0111986A2 and 0399108A1 which teach a two step process for preparing high molecular weight polyesters. A low molecular weight carboxylic acid terminated polyester prepolymer is first prepared and is then reacted with a diepoxide to further continue the chain building process. Each new polymer linkage formed between a carboxyl group of the prepolymer and an oxirane group of the epoxy resin consists of an ester group and a secondary alcohol. The alcohol serves as a locus for crosslinking which can be accomplished by thermally activated aminoplast or phenoplast curing agents. However, coatings made from such polymers may be subject to crazing, particularly after aging.